Optical spectrometers are devices that, when exposed to optical radiation, are able to ‘read’ the wavelengths of incident light and output a data set that can be used to tell a hardware device or a user the specific spectral content of that incident radiation. They can be used in a variety of applications, from academic to industrial settings, in research or in design. Generally, spectrometers rely on spectral separation of light via diffraction gratings, which is followed by detection via a coupled optical detector. Use of such gratings can involve extremely tight manufacturing tolerances, as well as extensive after-production qualification and calibration. Generally, commercial spectrometers using such gratings can cost thousands of dollars, and high-end spectrometers can cost tens of thousands of dollars.
Light incident on a diffraction grating is reflected off of the grating, such as at an angle dependent on the wavelength of the incident light. Differing wavelengths of light then spatially separate downstream of this grating, and are generally measured by a linear detector array. This spatially-resolved information is then converted to wavelength-resolved information using the geometry of the diffraction grating and the distance from the grating to the detector. To obtain high spectral resolution, the distance from the grating to the detector is generally quite long, resulting in physically large spectrometers.
One recent improvement to this diffractive grating approach is the use of the enhanced diffraction via the superprism effect in photonic crystals. However, this approach still scales in the same manner as a diffractive grating spectrometer. Smaller diffractive grating spectrometers generally sacrifice spectral bandwidth for spectral resolution.
For infrared (IR) measurements, Fourier transform infrared (FTIR) spectrometers are more commonly employed than diffractive grating spectrometers. In FTIR, measurements are spectrally resolved through the movement of mirror positions in an interferometer. The resolution is dependent on how precisely the mirror movements can be controlled. FTIR has the advantage of not requiring a detector array, but at the cost of requiring moving parts.